It is known how to activate carbon-containing raw materials. Different furnace types are used for this, but of these only the following three have attained general importance:
directly heated rotary furnace PA1 the multiple hearth furnace and PA1 the fluid bed furnace. PA1 Steam temperature: 600.degree. to 800.degree. C. PA1 Fluid bed temperature: 600.degree. to 950.degree. C. PA1 Temperature of the gas body: 950.degree. to 1200.degree. C.
The directly heated rotary furnace, which, because of its ruggedness, was very early on established for other high temperature processes, is probably the first large scale activation plant and that used most frequently. The transport of the carbonization means as well as of the energy takes place via the surface of the coal bed in contact with the gas body, while the removal of the reaction gases must take place over the same cross-section.
The surface is thus the size-determining factor of the rotary furnace technology, which can be influenced within certain limits by promoting the surface renewal (e.g. by increasing the rate of rotation and by provision of flights). (Cf. H. Helmrich et al., Chem. Ing. Techn. 51 (1979), 8, 771; J. K. Brimacombe et al., Metallurgical Transactions 9B, (1978), 201).
The multiple hearth furnace has only very recently been used for the production and re-activation of activated carbons (U.S. Pat. Ser. No. 3,994,829). It consists of a vertical, lined cylindrical housing, which is divided into separate hearths by lined trays.
The coal entering from above is transported in turn from inside to outside and vice versa over the trays and falls through holes onto the tray below. The horizontal transport of coal is accomplished by means of plough blades fixed to rotating arms. This also produces a certain surface renewal. The activating gases, heated to a high temperature, are introduced at the bottom and pass from hearth to hearth counter-currently to the coal. Each hearth can be heated by additional burners and/or additional steam or secondary air can be introduced.
The complicated mechanical arrangement of this multiple hearth furnace is at a disadvantage.
If one wants to increase the exchange surface between coal and reaction gas, flow of the gas through the coal bed suggests itself. This can be done in a fixed bed or a fluid bed.
The well known cylindrical fluid bed furnace, however, is not suitable for the activation of coals, since it has an unfavorable residence time spectrum. In processes where as high a throughput as possible is important (e.g. coal carbonization, waste combustion) the residence time behavior of the individual particle is not material. However, the activation process is a partial carbonization process, in which it is important (in order to achieve a homogeneous product) that each individual coal particle is as far as possible exposed to the same activation conditions (temperature, residence time and H.sub.2 O partial pressure). Uniform temperature distribution and good equalization of partial pressure because of the intensive mixing are the main features of the fluid bed. The irregular back mixing in the cylindrical fluid bed, however, has the consequence that only about 65% of the solid is discharged after the mean residence time, while 10% of the solid remains in the fluid bed for at least three times the residence time (Cf. H. Juntgen et al., Chem. Ing. Techn. 49 (1977) 2, MS447/77). Residence times of varying lengths, however, cause different degrees of combustion of the individual particles and thus an extension of the quality spectrum.
The decisive influence of the residence time spectrum in the activation of coal in the fluid bed can be seen from the fact that a 15-stage circular reactor is known for this purpose (J. Klein, Chem. Ing. Techn. 51 (1979) 4. MS680/79).
Various solutions have been proposed to avoid uncontrolled back mixing, e.g. dividing the reactor into several individual fluid beds, which are connected as cascades next to one another and located in a common furnace.
Another solution is known from U.S. Pat. Ser. No. 3,976,597. Here the fluid bed is in the form of a rectangular trough in which the fluidized coal--if possible in plug flow--move from the inlet to the outlet side.
The transport is accomplished by displacement of the material in the bed by the volume of coal fed in. The division of the trough into two or into several individual troughs has the purpose of narrowing the residence time spectrum even further.
The BV Fluid Bed Reactor (U.S. Pat. Ser. No. 4,058,374) represents a further development of this method. Here the individual troughs are arranged not next to one another but above one another, the fluidizing gas flowing through the individual fluid beds in turn. Inside the individual troughs back-mixing is to be prevented by baffles, which are immersed into the bed from above.
The disadvantage of this fluid bed technology is that the fluidizing gas on the one hand serves as reaction gas, but on the other hand has to carry the heat required for the endothermic carbonization reaction. Therefore only combustion gases can be used as a fluid medium, the temperature of which, corresponding to the heat requirement of the reaction, is higher than the activation temperatures in question. Also, the use of combustion gases as fluidizing gas means that water vapor partial pressures of at most 0.3 to 0.4 bars can be attained. To maintain the suspended state in the fluid bed a clearly defined minimum fluidization velocity is required, which is a function of the gas density (which changes with the temperature), the coal bulk density (which decreases during the course of the activation) and the coal particle diameter.
It may be gathered from the mutual dependence of the individual factors that an optimal setting is not simple with this fluid bed technology.
A further fluid bed reactor is known from German Offenlegungsschrift No. 2,615,437.
In this reactor, the harmful back-mixing effect is to be avoided by a greater height/diameter ratio and by feeding the coal through a tube immersed down to the grate and withdrawing it at the surface of the fluid bed.
Heating takes place indirectly through the jacket of the reaction space and by pre-heating the steam. The system permits working with very high water vapor partial pressures, at least over the gas inflow tray. When the fluid bed is high, however, there is pronounced formation of large bubbles (bypass flow), so that one has to operate with a large excess of steam. Furthermore, the additional heat transfer resistence of the reactor wall due to the indirect heating requires correspondingly high external temperatures.